A communication apparatus such as a radio base station includes a clock supplying device. The clock supplying device supplies a clock signal to be used in a signal process of the communication apparatus. The clock supplying device may include an oven controlled crystal oscillator (OCXO) as an oscillator generating the clock signal. The OCXO includes a crystal oscillator housed in a constant temperature oven, and generates and outputs the clock signal on a stable frequency.
The constant temperature oven is typically set at a relatively high temperature within a range of 70° C. to 80° C. to stabilize the oscillation frequency. A warm-up period may last from when the communication apparatus starts to be supplied with power from power on to when the constant temperature oven rises and reaches a specific temperature. During the warm-up period, the oscillation frequency remains unstable. While the OCXO is in a warm-up period, the communication apparatus may stop part or whole of the signal processing operation thereof.
In one method, a constant period of time from the power-on may be treated as a warm-up period in the warm-up control of the OCXO. In this method, however, the supplying of the clock signal remains stopped until the constant period of time has elapsed even if the warm-up operation is complete in practice. The power supplying may be interrupted and shortly restored. In such a case, the constant temperature oven may be already high when the power supplying resumes. The warm-up operation may be complete within a short period of time. Even in such a case, the communication apparatus is forced to wait on standby until the constant period of time has elapsed.
Another type of radio base station includes an OCXO and a temperature compensated crystal oscillator (TCXO). This type of radio base station compares a clock, into which an output of the OCXO is frequency divided, with a clock, into which an output of the TCXO is frequency-divided, and determines whether the OCXO has completed the warm-up operation in response to the phase difference. This arrangement shortens an excessively long warm-up waiting time.
Another type of clock supplying device has a duplicate structure to detect a fault therewithin. The clock supplying device measures a frequency difference between a reference clock input from the outside for system synchronization and each of the output clocks of an operative OCXO and a backup OCXO. The clock supplying device then identifies a fault in an operative clock, a backup clock, and the reference clock.